How to get consistent stroke thickness when zooming grease pencil?
It appears you can get something like this, by enabling "Volumetric Strokes" in your grease pencil palette.


This creates the following effect (reduced opacity for clarity, volumetric is on the right hand side).


The sampled points can be increased by adding subdivision (default is 0 for basic brush) under the stroke quality section in the grease pencil panel. Increased subdivision is only applied to new strokes though.


The volumetric strokes retain their size, regardless of viewport zoom.


Lastly, grease pencil objects are coming to blender 2.80, which will support proper scaling.